Solid materials with charged surfaces are used widely in the field of virus purification [Gagnon, P., Encyclopedia of Industrial Biotechnology: Bioprocess, Bioseparation, and Cell Technology, M. Flickinger Ed., J. T. Wiley and Sons, New York, 591-1611; Wolff, M. et al. Expert Rev. Vaccines 10:1451-1475 (2011); Gagnon, P., BioProcess Intl. Suppl. 6:24-30]. These materials commonly include so-called ion exchangers, which are usually employed in either of two application formats. In bind-elute mode, the sample and ion exchanger are equilibrated to conditions that allow the virus species of interest to bind. Contaminants that interact weakly or not at all with the charged surface fail to bind and are eliminated. Contaminants that interact more strongly than the virus bind more strongly. After washing to remove unbound contaminants, the column may be eluted by increasing the salt concentration. This permits fractionation of bound species in increasing order of the strength of their interaction with the ion exchanger, thereby achieving a high degree of virus purification. In flow-through mode, the sample and ion exchanger are both equilibrated to conditions that prevent the virus from binding. Species that interact more strongly with the ion exchanger than the virus are bound and thereby removed, but species that bind more weakly than the virus flow through with it and persist as contaminants. Both modes are performed on charged surfaces presented in a variety of solid phase architectures, including porous or non-porous particles packed in columns, or added directly to large volume aqueous samples, or on monoliths or membranes. These different architectures confer different flow properties, capacity, and resolution, but the defining chemical features of flow-through or bind-elute chromatography are constant regardless of physical format. Both methods rely on the equilibration of the ion exchanger and sample to the same conditions before the sample is introduced to the column.
Some chromatography media embody combinations of charges with other types of chemical functionalities. These media are broadly known as mixed-modes or multimodal materials, and may variously combine charges with hydrophobic groups, metal affinity, groups, or chemical groups that favor formation of hydrogen bonds. They are operated under different chemical conditions depending on the nature of the secondary functionalities, but they are still operated in bind-elute and flow-through mode.
A small minority of multimodal materials have been described that combine charge with a physical functionality, for example, where a charged material also has the ability to sort species according to their size. One such example employs variable size exclusion functionality in combination with an electropositive surface [Hunter, A., J. Chromatogr. A 897:65-80 (2000); Hunter, A., J. Chromatogr. A 897:81-97 (2000); Hunter, A., J. Chromatogr. A 930:79-93 (2000); Hunter, A., J. Chromatogr. A 971:105-116 (2000)]. The method generally involves entry of proteins into particle pores in a size-dependent manner while exhibiting codependence on the charge on the protein, as well as the buffer conditions. Like other charged materials for fractionation, the material described by Hunter et al is described exclusively for bind-elute and flow-through applications.